This invention relates to an improved oxidation process for the conversion of one or more aromatic hydrocarbon materials, having oxidizable substituents, to their corresponding acid derivative. More particularly, the process of this invention relates to a staged oxidation in an aqueous medium.
It is well known that aromatic hydrocarbons, having at least one, and preferably two or more, oxidizable substituent group may be converted into carboxylic acid products by effecting oxidation of such groups under controlled conditions. Such conditions have generally included the use of a known oxidation catalyst together with a suitable solvent such as a low molecular weight aliphatic carboxylic acid, such as acetic acid. A typical catalyst system comprises compounds of manganese and/or cobalt, together with a bromine-affording material. U.S. Pat. No. 3,092,658 describes the use of such a process for the continuous oxidation of substituted aromatic hydrocarbons to their corresponding carboxylic acid derivatives, particularly for the preparation of the isomeric phthalic acids from their xylene precursors in an acetic acid solvent.
In such an oxidation system a significant loss of acetic acid occurs, attributable to oxidation, handling losses, volatility, and the like. In order to avoid this economic penalty, attempts have been made to operate without a solvent but without significant success. A more promising approach has involved the use of a solvent system comprising water.
U.S. Pat. Nos. 4,258,209 and 4,278,810, commonly assigned, employ at least 5 wt % water, together with toluic, benzoic and/or acetic acid, as solvent, to provide an essentially homogeneous reaction mixture. More metal oxidation catalyst is required as the proportion of water increases. Processing may be batch or continuous but no bromine is employed. Acid yields are low, oxidation is incomplete, and purification of the acid product requires expensive processing.
U.S. Pat. No. 4,323,699 (and related European Patent Application 81302189.6) employs an aqueous medium and an iodine promoter. A heavy metal-bromine catalyst is present in a batch system and, again, yields of acid product are low while large quantities of intermediate oxidation products and by-products are obtained.
Earlier prior art examples include U.S. Pat. No. 3,012,038, wherein water is treated as the equivalent of lower aliphatic acids, aromatic hydrocarbons, halogen derivatives of benzene, and aromatic acids, as a suitable solvent under conditions selected to minimize corrosion of oxidation equipment. In another example, U.S. Pat. No. 3,711,539 permits water (an oxidation product) to vaporize from the reaction system to limit its concentration. Multi-state oxidation systems are disclosed in commonly assigned U.S. Pat. No. 3,064,044 and Canadian patent no. 704,424, each employing the conventional aliphatic acid solvent.
Further attention has been given to the use of an aqueous solvent system in such aromatic oxidations in an attempt to maximize the effectiveness of raw material oxidation, to lower the impurity level significantly to improve quality of product, and to provide an improved oxidation process from an economic viewpoint.